Voltage transients can induce very high currents and voltages that can penetrate electrical devices and damage them, either causing hardware damage such as semiconductor burnout, or electronic upset such as transmission loss or loss of stored data. The voltage transients produce large voltage spikes with high peak currents (i.e, over-voltage). The three basic over-voltage threats are electrostatic discharge, line transients, and lightning. Electrostatic discharge typically occurs when static charge dissipates off the body of a person in direct physical contact with an operating electronic system or integrated circuit chip. Line transients are surges in AC power lines. Line transients can also occur due to closing a switch or starting a motor. Lightning strikes can strike stationary objects, such as a building, or mobile objects such as aircraft or missiles. Such strikes can suddenly overload a system's electronics. At peak power, each of these threats is capable of destroying the sensitive structure of an integrated circuit chip.
Various overvoltage protection materials have been used previously. These materials are also known as nonlinear resistance materials and are herein referred to as voltage variable material. In operation, the voltage variable material initially has high electrical resistance. When the circuit experiences an overvoltage spike, the voltage variable material quickly changes to a low electrical resistance state in order to short the overvoltage to a ground. After the overvoltage has passed, the material immediately reverts back to a high electrical resistance state. The key operational parameters of the voltage variable material are the response time, the clamp voltage, and the voltage peak. The time it takes for the voltage variable material to switch from insulating to conducting is the response time. The voltage at which the voltage variable material limits the voltage surge is called the clamp voltage. In other words, after the material switches to conducting, the material ensures that the integrated circuit chip, for example, will not be subjected to a voltage greater than the clamp voltage. The voltage at which the voltage variable material will switch (under surge conditions) from insulating to conducting is the switch voltage. These materials typically comprise finely divided particles dispersed in an organic resin or insulating medium. For example, U.S. Pat. No. 4,977,357 (Shrier) and U.S. Pat. No. 4,726,991 (Hyatt et al.) disclose such materials.
Voltage variable materials and components containing voltage variable materials have been incorporated into overvoltage protection devices in a number of ways. For example, U.S. Pat. No. 5,142,263 and 5,189,387 (both issued to Childers et al.) disclose a surface mount device which includes a pair of conductive sheets and voltage variable material disposed between the pair of conductive sheets. U.S. Pat. No. 4,928,199 (Diaz et al.) discloses an integrated circuit chip package which comprises a lead frame, an integrated circuit chip protected by an electrode cover which is connected to ground on one side, and a variable voltage switching device including the voltage variable material connected to the electrode cover on the other side. U.S. Pat. No. 5,246,388 (Collins et al.) is directed to a device having a first set of electrical contacts that interconnect with signal contacts of an electrical connector, a second set of contacts that connect to a ground, and a rigid plastic housing holding the first and second set of contacts so that there is a precise spacing gap to be filled with the overvoltage material. U.S. Pat. No. 5,248,517 (Shrier et al.) discloses painting or printing the voltage variable material onto a substrate so that conformal coating with voltage variable material of large areas and intricate surfaces can be achieved. By directly printing the voltage variable material onto a substrate, the voltage variable material functions as a discreet device or as part of the associated circuitry.
It is commonly known in the art that the thickness of the voltage variable material and volume of the material are important to performance. See U.S. Pat. No. 4,977,357 issued to Shrier, U.S. Pat. No. 4,928,199 issued to Diaz et al. and U.S. Pat. No. 4,726,991 issued to Hyatt et al. Likewise, it is known that the clamp voltage is reduced or the voltage variable material can short out if put under pressure. See U.S. Pat. No. 5,248,517 issued to Shrier et al. Therefore, there has been a long felt need in the art to accurately and cost-effectively produce a variable voltage protection component having a uniform thickness of voltage variable material and to prevent shorts or variations in the clamp voltage if pressure is applied to the material. In addition to these qualities, it is desirable to have the voltage variable material be continuous across at least one of the surfaces of the variable voltage protection component for universal application of the component, for example, across a single circuit line or multiple circuit lines.
U.S. Pat. No. 5,262,754 (Collins) discloses an overvoltage protection element that can replace discrete devices presently used in protecting electronic circuits. The overvoltage protection element includes a layer of insulating material having first and second spaced major surfaces spaced a predetermined distance to determine the thickness of the element, a plurality of spaced holes extending between the major surfaces, and an overvoltage protection material contained within the holes formed in the layer of insulating material and extending between the spaced major surfaces. The spaced holes are formed by perforating the layer of insulating material by mechanical punching, laser processing and cutting, chemical etching, etc. The holes are formed in a pattern and should be wider than about one-half the width of the associated electrical circuit to which the holes will overlay. The spacing of the holes is determined by the spacing of the leads in the electrical circuit.
The above U.S. Patents referred to are incorporated herein by reference.
Although the prior art discloses various materials and devices, there is a continuing and long felt need to provide improved cost-effective voltage variable materials and devices of more consistent performance properties to prevent variations in the clamp voltage under various conditions in which the materials and devices are used.